Justin Bieber fielded an unusual request from a passenger on an international flight -- and it went something like this: shut your expletive-filled pie hole! Please.

It all happened during the Biebs' return from a Down Under promotional tour. Sources on the 12-hour Air New Zealand flight last weekend ... tell us JB was surrounded by his entourage in first class when things started getting out of hand.

We're told 18-year-old Justin and his posse were blurting out cuss words -- including some 4-letter selections -- and getting so loud ... a female passenger finally stepped to Bieber. We're told the woman ... a mother of 2 ... walked over and asked Justin to "stop yelling curse words and using that kind of language on a plane. It's not appropriate."

A source close to Justin tells us he was completely embarrassed, quickly apologized to the woman -- and then busted out an a cappella version of "Boyfriend."